(A) Except as provided in § 30.41 and division (B) below, an ordinance for the government of the city which has been adopted by the City Council without submission to the voters of the city shall not go into effect until 15 days after the passage of the ordinance.
(Neb. Rev. Stat. § 19-3701)
(B) In the case of riot, infectious or contagious diseases or other impending danger, failure of a public utility or any other emergency requiring its immediate operation, an ordinance shall take effect upon the proclamation of the Mayor and the posting thereof in at least three of the most public places in the city. The emergency ordinance shall recite the emergency, be passed by a three-fourths vote of the City Council and be entered of record on the City Clerk’s minutes.
(Neb. Rev. Stat. § 17-613)